Optical switches are devices that route optical signals along selected fibers of an optical network. Such switches constitute the fundamental building blocks of modern optical networks. Optical switch performance is judged by switching speed, optical insertion loss, operation lifetime, size and cost. Prior art optical switches typically operate on mechanical and opto-electronic principles. Many opto-electronic switches use birefringent elements and various types of reciprocal and non-reciprocal polarization rotators to accomplish their switching functions.
Recent network architecture has introduced a new switching function, namely optical multicasting where a signal is sent along multiple optical paths simultaneously. This function is required in addition to the conventional switching function of routing the optical signals along individual light paths. The new networks are specifically designed for applications such as audio and video conferencing, as well as software and video distribution. In multicasting, the sender transmits a message only once. At each intermediate node, copies of the message are made as required and sent out to each link. Multicasting is very cost effective compared with multiple unicasting, because it requires only one copy of the data-bearing signal at any given node.
Currently, copies of data in the optical domain are obtained by light splitting with power splitters or with the aid of star couplers incorporated with wavelength division multiplexing (WDM), wavelength tuning and selection techniques. Power splitters or star couplers enforce a rigid optical path architecture that is not reconfigurable. Therefore, multicasting has to be performed by incorporating wavelength manipulation techniques within these rigid optical path architectures.
Optical layer multicast schemes include all optical wavelength conversion to bands containing multiple wavelengths. In general, these schemes require wavelength multiplexing of the data channel on the transmitter side and distribution of the wavelength-division-multiplexing signal to every receiver. Each receiver is designed to be able to select any channel on demand. This means that the system has to use expensive WDM transmitters and/or tunable lasers and/or tunable filters as well as an optical amplifier. Meanwhile, there is usually a limit to the total number of usable wavelengths imposed either by total signal power or dispersion encountered in the nonlinear medium. Therefore, there are certain limits to the scalability of these schemes and the high power requirement may be another drawback in nonlinear optical schemes.
Prior art optical switches are primarily based on mechanisms that perform mechanical movements, change waveguide coupling ratios, and perform polarization rotations. Non-mechanical, solid-state optical switches are more desirable due to their intrinsic high-speed operation, excellent reliability and low power consumption. Many non-mechanical configurations have been reported based on mechanisms such as liquid crystal polarization rotation, thermal heating induced optical birefringence change, magneto-optic polarization rotation, and electro-optic retardation that changes either optical phase or polarization. Several exemplary non-mechanical optical switches and switching mechanisms including non-reciprocal Faraday rotators are discussed in U.S. Pat. Nos. 6,173,092; 6,392,784; 6,833,941; 6,867,895 and U.S. Application No. 2004/0027673.
Although these mechanisms can be modified for multicasting, the required alterations increase the number of moving positions in mechanical switches, and require more precise control electronics in all types of switches, consequently leading to drawbacks in speed and reliability. In addition, the complexity of the optical and electronic configuration and high losses will impose additional amplification requirements. For example, the waveguide based optical multicasting device described by Siyun Yu et al., “Lossless optical packet multicast using active vertical coupler based optic cross-point switch matrix”, IEEE, JLT, 2005 col. 23, p. 2984 require optical amplification due to intrinsically high loss deficiency and has to use the optical amplification. In addition, the fabrication costs for such a device are high.
For the above reasons, what is needed is a method and device for non-mechanical optical switching that can route an incoming light beam to multiple output ports simultaneously and on demand, while retaining the capability of conventional single port switching. In addition, such switches should be amenable to volume production. Moreover, in order to integrate these devices to compose the multicasting optical cross connects in scale, it would be particularly desirable to provide optical switches with low optical insertion loss and high speed switching that is reliable. Such devices should use fewer components of small size and require reduced alignment steps with large assembly tolerance to facilitate low cost manufacture.